In recent years, with the spread of portable electronic devices such as portable telephones, notebook personal computers and the like, there is a large need for development of compact and lightweight secondary batteries that have a high-energy density. Moreover, there is a large need for development of high-output secondary batteries as the power source for electric automobiles such as hybrid electric automobiles, plugin hybrid electric automobiles, battery-type electric automobiles and the like.
As a secondary battery that satisfies this kind of need, there are lithium-ion secondary batteries that are a kind of non-aqueous electrolyte secondary battery. This kind of lithium-ion secondary battery includes a negative electrode, a positive electrode, an electrolyte and the like, and a material that is capable of desorption and adsorption of lithium is used for the active material that is used as the material for the negative electrode and positive electrode.
Of this kind of lithium-ion secondary battery, a lithium-ion secondary battery that uses a positive electrode active material that includes a layered shape or spinel type lithium transition metal composite oxide for the positive electrode can obtain 4V-class voltage, so currently much research and development is being performed for a battery having high-energy density, and part of that research and development is being put into practical use.
As the positive electrode active material that is used for the positive electrode material of this kind of lithium-ion secondary battery, a lithium transition metal-containing composite oxide such as lithium cobalt composite oxide (LiCoO2) that is synthesized comparatively easily, lithium nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that uses manganese, lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2) and the like have been proposed.
Incidentally, in order to obtain a lithium-ion secondary battery that has excellent cycling characteristics and output characteristics, the positive electrode active material that is used for that positive electrode material preferably includes particles having a small particle size and narrow particle size distribution. Particles having a small particle size have a large specific surface area, and can sufficiently maintain a reaction surface area with the electrolyte. Moreover, by using particles having a small particle size, the electrode can be configured so as to be thin, so the distance that lithium ions travel between the positive electrode and negative electrode becomes short, and thus the electrode resistance can be reduced. On the other hand, by using particles having a narrow particle size distribution, the voltage that is applied to the particles inside the electrode is made uniform, and a decrease in battery capacity due to selective deterioration of fine particles is suppressed.
In order to further improve the output characteristics, forming spaces inside the positive electrode active material into which electrolyte can penetrate is effective. Positive electrode active material having this kind of hollow structure has larger reaction surface area than positive electrode active material that has about the same particle size but solid structure, so it is possible to greatly reduce the positive electrode resistance. The positive electrode active material is known to take on the particle characteristics of the transition metal-containing composite hydroxide that is the precursor to the positive electrode active material. In other words, in order to obtain a positive electrode active material having these excellent characteristics, it is necessary to suitably control the particle size, the particle size distribution and the particle structure of the particles of the precursor transition metal-containing composite hydroxide.
For example, JP2012246199 (A), JP2013147416 (A) and WO2012/131881 describe a method for manufacturing a transition metal-containing composite hydroxide as a precursor to a positive electrode active material by a crystallization process that is clearly separated into two stages, a nucleation process that mainly performs nucleation, and a particle growth process that mainly performs particle growth. In these methods, by appropriately adjusting the pH value and reaction atmosphere in the nucleation process and the particle growth process, a transition metal-containing composite hydroxide that includes secondary particles that have a structure that includes a low-density center section that includes fine primary particles having a small particle size and narrow particle size distribution, and a high-density outer-shell section that includes flat-shaped or needle-shaped primary particles.
Moreover, WO2014/181891 describes a method for manufacturing a transition metal-containing composite hydroxide that is characterized by including a nucleation process for performing nucleation by controlling the pH value of a nucleation aqueous solution that includes a metal compound that includes has at least a transition metal and an ammonium ion donor to be 12.0 to 14.0, and a particle growth process that causes nuclei to grow by controlling the pH value of a particle growth aqueous solution that includes the generated nuclei to 10.5 to 12.0 that is value lower than the pH value in the nucleation process, and the atmosphere is controlled so that together with the nucleation process and the start of the particle growth process being a non-oxidizing atmosphere, at least one time the atmosphere is switched to an oxidizing atmosphere at specified timing in the particle growth process, after which the atmosphere is switched again to a non-oxidizing atmosphere. With this method, a transition metal-containing composite hydroxide that includes secondary particles having a small particle size and narrow particle size distribution and having a structure that has a center section that is formed by an aggregate of plate-shaped or needle-shaped primary particles, and at least one layered structure that includes at low-density layer that is formed around the outside of the center section by an aggregate of fine primary particles and one high-density layer that is formed by an aggregate of plate-shaped primary particles, with the layers being alternately layered.
The positive electrode active materials that have these transition metal-containing composite hydroxides as a precursor are composed of particles having a small particle size and narrow particle size distribution, and that include a multi-layered structure having hollow structure or spaces. Therefore, in a secondary battery that uses these positive electrode active materials, it is considered possible to simultaneously improve battery capacity, output characteristics and cycling characteristics. However, the manufacturing methods that are described in the literature above require time to switch the reaction atmosphere in the particle growth process, so during that time, it is necessary to temporarily stop the supply of the raw material aqueous solution and the like, so there is room for improvement from the aspect of productivity. Moreover, for a positive electrode active material that is used in a secondary battery that is used as an electric power source of an electric automobile, there is a need for further improving the output characteristics thereof without impairing the battery capacity or cycling characteristics of the secondary battery.